Genome-wide association studies identified a nonsynonymous SNP (rs16969968) within the CHRNA5 gene that encodes for the ?5 nicotinic receptor (nAChR) subunit. This SNP produces a twofold higher risk for heavy smoking and increases the risk for lung cancer. Consistent with the human genetics, our preliminary studies showed that the ?5 nAChR is necessary for the expression of the nicotine withdrawal syndrome. The ?5 nAChR also regulates sensitive to nicotine-induced behaviors and controls nicotine self-administration at high doses. In these proposed studies, we will investigate ?5's mechanistic action on the midbrain dopamine (DA) systems that reinforce rewarding and addictive behaviors. We will examine the effect of the rs16969968 SNP on DA signaling from its source in the midbrain to its main targets in the striatum, including the nucleus accumbens (NAc) which is important for processing reward. Our in vivo multi-tetrode recordings show that nicotine increases the phasic burst firing of DA neurons, and our cyclic voltammetry and in vivo microdialysis data show that nicotine-induced changes in DA neuron firing are translated in a target-specific manner in areas that process reinforcement and reward, including the NAc core and shell. However, little is known about the role of the ?5 subunit or the rs16969968 SNP and about the role of the ?5-subunit in regulating the relationship between DA neuron firing and DA release in targets. Our working hypothesis is that nicotine acts via the ?5-nAChR subunit to modulate DA signaling both at the source (i.e., DA neurons in the midbrain) and at the targets (including the NAc and the dorsal striatum). The aims examine the hypothesis that nicotine-induced changes in the DA system evolve during chronic nicotine exposure and during the withdrawal period. The significance of the study originates from the expectation that these nicotine-induced activities and changes in the DA system contribute to the transition from initial nicotine use to addiction. Tobacco use remains the leading cause of preventable death in the United States, and these studies provide a mechanistic basis for nicotine addiction and for developing therapies to aid smoking cessation.